1. FIELD OF THE INVENTION
The invention relates to the utilization of a terpolymer of: (1) a monovalent alkali metal salt of acrylic acid; (2) an hydroxy alkyl acrylate; and (3) acrylamide, to effectively reduce the filtrate loss of an aqueous drilling fluid used in the drilling of a subterranean well.
2. DESCRIPTION OF THE PRIOR ART
It is generally agreed among those skilled in the art that a rotary system is a most acceptable form of drilling an oil or gas well. This system depends upon the rotation of a column of drill pipe to the bottom of which is attached a multi-pronged drilling bit. The bit cuts into the earth causing the cuttings to accumulate as drilling continues. As a result, a drilling fluid must be used to carry these cuttings to the surface for removal, thus allowing the bit to continue functioning and the bottom hole to be kept clean and free of cuttings at all times. Drilling systems other than the rotary system are sometimes employed during drilling operations. Nevertheless, these systems still require a drilling fluid to remove the bore hole cuttings and to otherwise perform functions related to drilling fluids.
Oil-producing formations are generally porous layers having varying degrees of permeability to the flow of fluids such as oil, water, or gas. Consequently, the rate of oil production is largely determined by the rate of flow through these permeable formations which, in turn, is dependent upon the porosity or permeability of the sand or stone present. In drilling through such a porous layer, it is desirable to employ a drilling mud having such characteristics that excessive amounts of liquids or solids are prevented from penetrating the porous formation. The ability of the drilling mud to prevent excessive formation fluid penetration is called filtration control.
Materials that have been used in the past to control filtration rates of aqueous drilling fluids by plugging, producing cakes, or similar methods, have included materials such as pre-gelatinized starch, sodium carboxylmethylcellulose, sodium polyacrylates, and lignites. Each of these materials have certain limitations. For example, lignite becomes ineffective in high salt concentrations.
Acrylic and methacrylic derivatives, such as those which are co-polymerized with hydrocarbon substituted styrenes, such as alpha methyl styrene, para methyl styrene, 2-4 dimethyl styrene, and the like have been utilized in drilling fluids. For example, U.S. Pat. No. 2,718,497, to Oldham, et al, teaches the use of relatively high molecular weight polymers of these materials to control water loss characteristics of aqueous muds and clay dispersions. Additionally, U.S. Pat. No. 2,650,905, to Fordyce, et al, teaches the use of water soluble sulfonated polystyrene derivatives for filtration control in water-based drilling fluids.
The prior art has utilized acrylic acid derivatives as thickeners for numerous commercial purposes, including utilization in drilling fluids. For example, U.S. Pat. No. 4,059,552, to Zweigle, et al, teaches the use of acrylamide-sodium acrylate or acrylic acid-substituted acrylates. A similar thickening material is disclosed in U.S. Pat. No. 4,037,035, to Blanc, et al, by utilization of an acrylamide-sodium acrylate constituent with an acrylamide-acrylic acid alkanolamine. Similarly, copolymers of acrylamide and sodium acrylate and acrylate derivatives thereof formed by irradiation polymerization are utilized as thickeners, as disclosed in U.S. Pat. No. 3,926,756 to Restaino. U.S. Pat. No. 3,897,404 to Korte, et al, teaches utilization as thickeners for printing paste and the like of substituted acrylamide-acrylic acid-acrylate derivatives.
Hydrophilic gels derived from 2-hydroxyethyl methacrylate have been found to be useful in a number of medical applications as material for gel filtration, such as copolymers of acrylamide, acrylic ester-2-hydroxyethyl methacrylate, as disclosed in U.S. Pat. No. 3,948,841, to Dusek.
Acrylamide-sodium acrylate-2-hydroxyethyl acrylate crosslinked agents of a comparatively high molecular weight are utilized as soil stabilizers as disclosed in U.S. Pat. No. 3,651,002, to Higashimura, et al.
Acrylic acid derivatives such as copolymers of acrylamide and sodium acrylate derivaties have been frequently and commercially utilized as flocculants for drilling fluids, and are disclosed in U.S. Pat. Nos. 3,558,545, and 3,472,325 to Lummus. Similarly, a copolymer derived from acrylic acid and acrylamide is disclosed in U.S. Pat. No. 3,323,603 to Lummus et al, as a flocculant for aqueous drilling fluids.
In the present invention, enhanced filtration control is obtained by incorporation into an aqueous drilling fluid of a filtration control agent consisting essentially of a terpolymer of: (1) a monovalent alkali metal salt of acrylic acid; (2) an hydroxy alkyl acrylate; and (3) acrylamide. Although it is not fully understood, it is believed that the terpolymer will produce a filter cake along the bore hole to maintain effective filtration control during circulation of the drilling fluid within the well.
It is an object of this invention to provide a filter cake which is substantially unaffected by comparatively high bore hole temperatures and pressures.
It is a further object of this invention to provide a filtration control agent which will not materially affect the viscosity of the utilized drilling fluid system.
It is a further object of this invention to provide a comparatively low cost, easily prepared, filtration control agent for use in aqueous drilling fluids.
It is a further object of this invention to provide a filtration control agent for aqueous drilling fluids which is comparatively insensitive to salt environments and concentrations within the aqueous system.
It is a further object of this invention to provide a filtration control agent which may be utilized in highly weighted drilling fluids.
Other objects and advantages of the present invention will be easily appreciated by those skilled in the art from a reading of the description, examples and claims which follow.